1. Field of the Invention
This invention relates to a heat driven thermoacoustic motor, or sound generator, powering either a thermoacoustic refrigerator or an electricity generator. Specifically, the invention consists of a geometry and other optional components that improve the efficiency, power density, and ease of start-up for a thermoacoustic motor. Applications include remote or recreational (e.g. camping) or solar power requirements for cooling or electricity; or for residential or commercial cooling where the cost of fossil fuel is much lower than the cost of electricity or peak electrical loads need to be reduced. The heat driven cooling engine invention can be scaled down to very small sizes for the purpose of cooling integrated circuits or sensors.
2. Description of the Related Art
While some features and advantages of this invention depend of convenient optimization and adjustment of thermoacoustic parameters, some of the advantage derives from capitalizing on the potential for additional Stirling cycle heat transport in addition to the primary thermoacoustic heat transport.
Both the Stirling cycle and the thermoacoustic cycle share many features and a common parameter space. Both utilize a primary thermodynamic fluid medium (usually a gas) that executes a reciprocating motion and an oscillatory pressure. The primary thermodynamic medium is in thermal contact with a secondary "recuperative" thermodynamic medium (usually a solid). The secondary solid medium has pores or channels that allows fluid flow with little resistance. The primary medium converts the work done by the pressure change of the primary medium into some combination of a temperature change or a heat flow to or from the secondary medium. The function of the secondary medium is to alternately sink and store heat on one half of the cycle and then source and release heat on the other half of the cycle. The heat storage capacity of the secondary medium is very much larger than that of the primary medium.
The differences between the Stirling cycle and the thermoacoustic cycle are as follows. In an unlimited thermally diffusive medium, heat can diffuse about one "thermal penetration depth" in distance in the time of one oscillatory cycle. The secondary medium in a Stirling engine is called a "regenerator" and has pores or channels that are very small relative to a thermal penetration depth, thus largely suppressing any temperature oscillation of the fluid. The primary "rule" for the Stirling cycle is that heat transport in the regenerator is equal in magnitude, and opposite in direction to the mechanical power flow (or acoustic power flow) in the fluid.
Thus a regenerator in a perfect standing wave (pressure and velocity phasing is in quadrature) can sustain a large temperature difference from end to end but will transport no heat, since there is no acoustic power flow. This was the original intent of the regenerator in the Gifford & Longsworth "Pulse Tube Refrigerator", U.S. Pat. No. 3,237,421 (March 1966). in reality, the power flow in this regenerator was not zero and a substantial amount of beneficial heat transport was achieved by both the pulse tube and the regenerator. In the orifice pulse tube refrigerator (Radebaugh, "A comparison of three types of pulse tube refrigerators: New methods of reaching 60K", Advances in Cryogenic Engineering, 31, 779 (1986)) the amount of mechanical power flow in the regenerator is increased by the dissipative orifice which increases the regenerator's heat transport.
The secondary medium of a thermoacoustic engine is called a "stack" and its pore or channel size is comparable to a thermal penetration depth, which allows the fluid temperature to oscillate, while allowing imperfect thermal contact between the two media. The stack transports heat in a perfect standing wave, where phasing between pressure and velocity are in quadrature, because of the time delays or phasing of the thermal diffusive wave in the stack's pores or channels. Creating the standing wave environment for the stack is trivial compared to the traveling energy wave required for the regenerator, thus achieving the mechanical simplicity of the typical thermoacoustic engine. The direction of heat flow in a refrigerating stack is always towards the nearby pressure antinode (PAN) of the standing wave (where the acoustic pressure distribution is at a maximum and the acoustic velocity is at a minimum or zero), and directed away from the PAN for a prime mover (motor).
However, the standing wave in a thermoacoustic engine is never perfect, and there is always acoustic energy flow through the stack. Because of this energy flow, there is always a Stirling component to the heat transport, in addition to the thermoacoustic transport. While the magnitude of the Stirling component may be small compared to the thermoacoustic component (perhaps 10%), it is either contributing to the performance of the engine (additive) or detracting (subtractive). So if the Stirling component of heat transport is managed properly in a thermoacoustic engine design, it will generally result in at least a 20% improvement in the efficiency, relative to an engine having a subtractive Stirling component.
In the original Wheatley thermoacoustic refrigerator, (Wheately, et al., "Acoustical heat pumping engine", U.S. Pat. No. 4,398,398 (Aug. 16, 1983)), the acoustic power source (the electromagnetic driver) is situated some distance from the cold end of the refrigerating stack. Thus acoustic power flows from the cold end of the stack to the warm end of the stack. The resulting Stirling component of heat transport is oriented from warm to cold and thus degrades the efficiency of the refrigerator.
In the second generation thermoacoustic refrigerator by Hofler, (Hofler et al., "Acoustic Cooling Engine", U.S. Pat. No. 4,722,201 (Feb. 2, 1988)), the acoustic power source (i.e. the driver) is situated at a PAN near the hot end of the refrigerator stack. The acoustic resonator is effectively 1/4 of a wavelength, with large spherical volume at the end opposite the driver. The spherical volume approximates an open-end acoustic condition for the resonator. One reason for this invention was to prevent conduction or acoustic convection of heat from the warm driver to the cold end of the stack. However, this also orients the direction of the Stirling component of heat transport from the cold end of the stack to the warm end, thus improving the efficiency of the refrigerator.
In the heat driven thermoacoustic cooling engine by Wheatley et. al., "Heat-driven Acoustic Cooling Engine Having No Moving Parts", U.S. Pat. No. 4,858,441 (Aug. 22, 1989), the usual thermoacoustic prime mover is combined with the second generation thermoacoustic refrigerator, all combined in a 1/4 wavelength portion of the standing wave (from PAN, to velocity antinode or VAN). This engine is shown schematically in FIG. A. While this engine is simple and has no moving parts, it has very poor efficiency, with a total COP in the range of 0.05 to 0.1 for a modest cooling temperature span. COP is an acronym for "coefficient-of-performance." Total COP is the cooling power divided by the heat input power, which can be much greater than unity for the combination of two Carnot cycle engines; a motor and refrigerator combination having perfect thermodynamic efficiency. There are two primary reasons why the efficiency is so low.
In FIG. A, the cooler stack pumps heat Q upward, while the acoustic power W comes from the prime mover stack above it, creating a downward acoustic power flow. Thus the Stirling component of heat transport is directed upward enhancing the efficiency of the cooler stack. However, heat flows downward through the prime mover stack from hot to ambient temperature and the Stirling component of acoustic power flow is thus directed upward in opposition to the thermoacoustic component of mechanical power provided to the cooler stack. This substantially degrades the efficiency of the prime mover stack.
The second disadvantage of the old design is that the prime mover and cooler stacks must fit into mutually exclusive regions of the standing wave. Typically, for small temperature span thermoacoustic engines, the stack performs best when it occupies a region in the standing wave extending from 0.08 to 0.2 radians, measured from the pressure antinode (PAN). This is more or less true for both prime mover stacks and for cooler stacks. Hence, in the original design of Wheatley U.S. Pat. No. '441, the prime mover stack is too close to the PAN and the cooler stack is too far from the PAN for optimal performance.
While the two stacks in FIG. A could have different diameters, this would be difficult to accomplish without increasing the undesirable distance between the stacks. Also, it would not have the desired effect of altering the power balance between the two stacks. Increasing the diameter of one stack would increase the cross-sectional area of the stack but would decrease the acoustic velocity. The two effects would roughly cancel, leaving the power capacity of the stack unchanged.
Another heat driven cooler invention is the thermoacoustically driven orifice pulse tube refrigerator, patented by Swift et al, "Acoustic Cryocooler", U.S. Pat. No. 4,953,366 (Sep. 4, 1990). This is an effective and moderately efficient cryocooler engine having no moving parts. However, the orifice pulse tube refrigerator is only efficient in the cryogenic temperature regime and is inherently inefficient for small temperature spans. The orifice pulse tube refrigerator is essentially a Stirling engine variant where the cold temperature cooling power is comparable in magnitude to the mechanical power dissipated as heat in the orifice. For cryogenic coolers where even the Carnot COP is very small, the energy wasted by the orifice is almost negligible and less important than other factors. For a small temperature span refrigerator, such as an air conditioner, the orifice pulse tube refrigerator, by itself, would have a COP less than unity. And a heat driven version would have a total COP very much less than unity. This is a prohibitively poor efficiency.
The heat driven cooler topology disclosed in this invention allows independent positioning of the two engine stacks with respect to the standing wave, and also aligns the acoustic power flow and the heat flow to be anti-parallel in both stacks for best efficiency. The topology also allows the power capacity (heat power or acoustic power as the case may be) of each stack to be independently adjusted by changing the diameter of each stack. This is advantageous for efficiency optimization in general, and for ensuring easy engine start-up with a modest hot temperature.